It depends on the cause of the obstruction. For crohn's there are two to worry about.
First being inflammatory which is FAR more common. This is where your intestines swell and simply squeeze themselves shut. It will be painful and everything and it will usually resolve itself when the inflammation decreases. Like in my case my inflammation ebbs and flows all the time so my intestinal flow rate varies a lot. I still manage to get enough out over time to avoid the hospital though. It's horrible, miserable, but usually not an emergency. Steroids are good for this.
Second is a stricture (narrowing) of the intestines by scar tissue. This isn't something that inflates and deflates and it's not nice soft tissue that will give up it's grip when enough pressure is exerted. Scar tissue is hard and permanent. Combined with inflammation or not it can be enough to completely cut off your intestines from flowing. When you haven't gone at all for 3 or 4 days it's ER time. Even earlier if you're not even passing gas at all. Scar-based strictures have to be cut out.
The symptoms to look for in our case (since we're in pain all the time anyway) are things like bad breath/taste (You'll be able to smell/taste the backed up crap and gas) and vomiting (especially if it seems like fecal matter coming up). As far as signs, pay attention to how much is going in and coming out and if things aren't adding up right compared to before you suspected an obstruction.
One very important thing to remember is to never take stimulant laxatives. Stimulant laxatives can take an obstruction and put enough pressure behind it to make your intestines burst. Not good. Stool softeners and non-stimulant laxatives are great though because they'll make it softer so that it can conform better to the space available or lubricate it so that it can push through the bottle-necks.
